It is known that different aerofoil shapes for fan blades, or compressor blades, perform differently at different operating conditions. A fan blade, may be optimised for take off conditions, or for cruise conditions, but to perform adequately for both take off conditions and cruise conditions generally requires a compromised aerofoil shape. The compromise to the aerofoil shape is based upon the mission that a particular gas turbine engine is to perform, for example whether the gas turbine engine is for long distance flight, short distance flight etc.
A fan blade, or a compressor blade, generally comprises a twisted aerofoil, e.g. the stagger angle increases from the root to the tip of the aerofoil portion of the fan blade or compressor blade, to give the correct angle of incidence at each point along the length, radius, of the aerofoil portion of the fan blade, or compressor blade. In operation due to rotation of the rotor upon which the fan blades, or compressor blades, are mounted the aerofoil portions of the fan blades, or compressor blades, untwist due to the centripetal loads and these centripetal loads increase with increasing rotational speed. Typically the aerofoil portion of a fan blade untwists by about 5°, where 4° of the fan blade untwist is due to the centripetal loads and 1° of the fan blade untwist is due to gas pressure loads. Generally the increase of untwist with increasing rotational speed is an advantage because it results in an increase in efficiency, but the untwist of the aerofoil portions of current fan blades does not provide as much untwist as is required.
A fan blade, or compressor blade, generally comprises an aerofoil portion, which is strongly curved, or cambered, into a C-shaped cross-section near the root portion of the fan blade, or compressor blade, because the gas flow velocity is relatively low and the curvature of the aerofoil portion enables the gas flow impinging on the radially inner region of the aerofoil portion to work harder on the inner region of the aerofoil portion. The aerofoil portion 138 is nearly straight in cross-section in the tip region of the aerofoil portion 138, with a suction surface 148, a pressure surface 150 and a bend at the trailing edge 146, as shown in FIG. 11. The straightness and amount of bend at the trailing edge 146 of the aerofoil portion 138 is a matter of compromise between part speed, e.g. cruise, and high-speed, e.g. take off, operational requirements.
A fan blade, or compressor blade, optimised for high-speed, e.g. take off, operation is S-shaped in cross-section at the tip region of the aerofoil portion 238 as shown in FIG. 12. An S-shaped cross-section is generally one where the suction surface 248 is initially concave 247 and then convex 249 between the leading edge 244 and the trailing edge 246 and the pressure surface 250 is initially convex 251 and then concave 253 between the leading edge 244 and the trailing edge 246.
A compressor blade, optimised for part speed, e.g. cruise, operation is C-shaped in cross-section at the tip region of the aerofoil portion 338 as shown in FIG. 13. A C-shaped cross-section is generally one where the suction surface 348 is convex between the leading edge 344 and the trailing edge 346 and the pressure surface 350 is concave between the leading edge 344 and the trailing edge 346.
High speed operation, for example take off, is 100% rotational speed and part speed, for example cruise, is 95% rotational speed, although it is desirable to reduce the air speed of an aircraft during cruise conditions to increase the efficiency of the aircraft and this results in a corresponding reduction in rotational speed of the rotor and fan blades and therefore cruise speed may be less than 95% rotational speed.
Fan blades, which can be optimised for a wide variation in operating rotational speeds have a significant advantage to enable a significant reduction in specific fuel consumption.